Experimental Study On Viscoelastic Properties Of Human Periodontal Ligament By Dynamic Mechanical Analysis

ObjectiveIt is an important role that the periodontal ligament(PDL)plays in orthodontic tooth movement.When the prolonged mechanical force is applied to a tooth,the periodontal ligament transmits the load to alveolar bone inducing the socket remodeling.For a better understanding of orthodontic tooth movement,it is necessary to study the biomechanical properties of the PDL.In current study,we took advantage of Dynamic Mechanical Analysis(DMA)to study the viscoelastic properties of human PDL under dynamic tension and compression load,which would be complementary to the numerical simulation of orthodontic tooth movement.The aim of this study is to measure the viscoelastic properties of the human PDL using dynamic mechanical analysis and to determine the different response related to frequency,tooth position,region and age.Material and methodsPart I,the dynamic tensile test of human periodontal ligament: This study was carried out on three human maxillary jaw segments.Transverse sections of 2mm thickness were cut perpendicularly to the root longitudinal axis of maxillary central and lateral incisors,and bar-shaped samples were extracted from these sections with 8×4×2mm dimensions.DMA was used to investigate the mechanical response of the human PDL.The dynamic sinusoidal load with amplitude of 3N,resulted an approximate strain of 0.2,were under the frequency between 0.5Hz and 10Hz(0.5 Hz,1 Hz,2 Hz,5Hz,10Hz)for 15 minutes.All samples were grouped by tooth positions and longitudinal regions.The differences between each group were compared by a multi-factor analysis of variance(MANOVA).Part II,the dynamic compressive test of human periodontal ligament: This study was carried out on three human maxillary jaw segments.Transverse sections of 2mm thickness were cut perpendicularly to the root longitudinal axis of maxillary central and lateral incisors,and cube-shaped samples were extracted from these sections with2×2×2mm dimensions.Frequency sweep tests were performed under the selected frequency between 0.05 Hz and 5Hz(0.05 Hz,0.1 Hz,0.2 Hz,0.5 Hz,1 Hz,2 Hz,5Hz)with a compression amplitude of 2%.The maximum strain of PDL was set to be 8%.Each specimen was tested in triplicate for 15 minutes each.All samples were grouped by tooth positions.The differences between each group were compared by a multi-factor analysis of variance(MANOVA).ResultsIn the dynamic tensile test,the average storage modulus ranged from 0.808 MPa to 7.274 MPa,the loss modulus varied from 0.087 MPa to 0.891 MPa,and the tan varied from 0.1416 to 0.1494.With the increase of oscillation frequency resulted,storage and loss moduli of the PDL changed markedly in the frequency range of0.5Hz and 2Hz and then kept steady from 2Hz to 10 Hz.The tan,representing theratio between viscosity and elasticity,remained constant with frequency.The trends for storage and loss moduli were described by exponential fits.The dynamic moduli of central incisor were higher than those of lateral incisor.The PDL samples from the gingival third of the root showed the lower storage and loss moduli than those from the middle third of the root.In the dynamic compressive test,the storage modulus ranged from 37.20 MPa to655.98 MPa,the value of loss modulus was between 8.01 MPa and 117.73 MPa,and the tan varied from 0.1315 to 0.4258.The storage increased markedly withfrequency.The loss modulus increased with frequency from 0.05 Hz – 0.5Hz and remained constant after 0.5Hz.The tan,the ratio between the storage and lossmoduli,showed a negative logarithm correlation with frequency.The dynamic moduli and the loss tangent of central incisor were higher than those of lateral incisor.Conclusion1.This study demonstrated the human PDL is viscoelastic through the range of the frequency tested.The elastic component is the dominance of mechanical response.2.The viscoelastic relationship changed with respect to frequency,tooth position and root level.3.The dynamic moduli and loss tangent from compressive test were higher than those from tensile test,which indicated that the PDL showed more viscosity and stiffness under compression load.